1. Field of the Invention
The present invention relates to technology for controlling timing of turning on a plurality of light emitting units in an optical scanning device for use in an image forming apparatus.
2. Description of the Related Art
In a field of an image forming apparatus such as a laser printer and a digital copier, a demand is growing for improving an image forming speed (high speed) and writing density (high image quality). To meet the demand, one known approach is to scan a scan target surface with a plurality of light beams by using a multibeam light source capable of emitting a plurality of light beams simultaneously.
The multibeam light source includes an array of light emitting units such that a certain light emitting unit is displaced from an adjacent light emitting unit in a direction corresponding to a main-scanning direction. When the light source is configured in this manner, it is possible to narrow an interval between adjacent scanning lines. In this arrangement, however, timing of turning on each light emitting unit needs to be controlled to align writing start positions on a plurality of scanning lines.
For example, Japanese Patent Application Laid-open No. 2003-276234 discloses an image forming apparatus that includes a control unit that performs of/off control of the light emitting units. The control unit determines timing of turning on at least one light emitting unit of a light source based on an image clock generated in synchronization with a signal detected by a synchronization detecting unit, and also timing of turning on other light emitting units by changing a count value of the image clock, thereby controlling turn-on of the light emitting units.
In a typical image forming apparatus, optical characteristics of optical components or a positional relationship between optical components may change due to temperature change, temporal change, or the like after shipping. Such a change in the optical characteristics causes displacement of a light spot position from a desired position on a scan target surface. This, for example, results in displacement of a writing position of image data in a main-scanning direction. In the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2003-276234, it is difficult to correct the displacement of the writing position with high accuracy in real time.